Thin clients are computing devices that are connected to a server. In addition to having methods for input and output, thin clients include some limited microprocessor and operating system capability. They are called thin clients because they run “thin” operating systems on their microprocessors, and are not able to implement complex or intensive programs in a timely manner. They are thin compared to “fat” operating systems of a networked or stand-alone computer, which generally has much more processing capability in its microprocessor and operating system.
Normally thin clients include an input device, such as a keyboard and/or mouse, and an output device, such as a display monitor. Depending on what function the thin client serves, these devices could be combined into a single device, such as a touch screen, or may all be separate. Thin clients are typically deployed when the processing power of a typical computer is not necessary, and/or when a less expensive solution will meet requirements of a particular installation.
Functions of the thin client processor include, for example, sensing an input on the input device, communicating the input to the server, receiving codes and data that form an output display image from the server, and causing the output device that is connected to the thin client to display the desired image. Also, at least some memory is usually included in the thin client, for storing data and commands received by the server, for a de-compression work area, and for other uses. Additionally, some sort of long-term storage, such as a hard disk drive, may be present in the thin client.
Thin clients differ from typical standalone or networked computers in that thin clients typically do not run application programs directly on their microprocessors, whereas standalone and networked computers generally do run them. Programs that appear to be running on a thin client actually run on the server to which the thin client is connected. The thin client simply acts as a pass-through, accepting input from the keyboard and/or mouse, passing the input through to the server, and displaying on its screen what the server sends it to display. Typical personal computers in a network setting, conversely, run application programs directly on their own processor.
As an example of this difference between a thin client and a networked computer, consider a word processing application running on both machines. On the networked computer, little or none of the processing necessary to run the application is done on the network server. Keyboard input is detected and processed by the networked computer, data is stored in local memory and probably cached out to a local hard drive, and the display is also controlled locally.
On a thin client, conversely, the actual word processing program runs on the server and the server's memory stores the necessary programs and data. Data that is input on the thin client keyboard is transferred to the server for any necessary operation. The server then decides what the thin client will show on its screen, and sends the exact data and/or commands to the thin client to cause it to show the desired image.
Thin clients communicate with the server through a communication channel. Communication protocols used between thin clients and servers differ from those used between networked computers, in order to take advantage of the special function of the thin client. Communication protocols between the server and the thin client are commonly called remote desktop communication protocols. As used throughout this description, discussion of remote desktop communication protocols means a family of protocols used to communicate between servers and thin clients, or between more than one thin client, and not any one particular protocol. Because bandwidth is usually limited between a thin client and its server, the communication protocols include mechanisms for data compression.
Newer applications require thin clients to display increasingly more complex image displays, such as audio and video applications. A video application can be thought of as a sequence of still pictures that are shown in rapid succession. Because the human eye briefly “retains” a copy of an image it perceives, a series of images is interpreted as a continuous video if the successive frames are shown quickly enough. Normally, humans fail to notice any discontinuity in the video if the successive frame images are shown more than 20 times per second. Typical applications try to show video images at a rate of 30 frames per second, so that no discontinuities are perceived, even if the framerate is somewhat slowed. As imagined, sending a series of pictures to a thin client in order to create a video sequence requires a large amount of data sent from the server.
Although current remote desktop communication protocols include some data compression techniques, these techniques fail to reduce the amount of data sent from the server to a thin client to a level adequate to ensure that video playback on the thin client will operate correctly, given the low bandwidth requirements of the communication link. Adequate playback is achieved when a video is displayed on the thin client without slow screen updates, dropouts, artifacts or other problems that distract the viewer. Many thin client systems fail to achieve adequate playback due to the bandwidth constraints and the way in which that bandwidth is used.
The present invention addresses this and other problems associated with the prior art.